The present invention relates to inverter circuits for converting direct current power into alternating current power. More specifically, it relates to inverter circuits utilizing field-effect transistors for increasing overload driving capability.
DC-AC inverters are designed to convert a DC power source into an AC power output. Such inverters are commonly used to provide power for AC induction motors, to light incandescent lights, or the like, from a battery or gasoline powered electric power source. It is important in many such applications that the inverter have the ability to handle brief overload current demands. For example, a lamp may require ten times its normal running current to start up.
In the earlier art, the battery power source was coupled to an electric motor/generator combination called a dynamotor to provide AC power. Such an arrangement required considerable current to operate, and was therefore not very efficient. Early electronic DC to AC inverters used a large, low frequency transformer. A vacuum tube or transistor switching arrangement induced a current in the primary of the transformer and an alternating current was thereby induced in the transformer secondary. A disadvantage of using this type of DC to AC converter was the bulk and weight of the transformer. In addition to being expensive to manufacture, the transformer was inefficient and tended to generate heat and excessive vibration. Often, the value of voltage produced, both as to voltage level and frequency, was also unstable.
Transformers and rotating electrical machines have traditionally had substantial short term overload capability, since an overload simply dissipates more power in the copper which make up these machines. However, this ability to dissipate excess heat energy does not exist in bipolar transistor devices, thyristors, and other electronic switching devices used in more recent solid-state DC-AC inverters. A significant problem with such prior art solid-state DC-AC inverters is that the absolute maximum rating at which the inverter can be operated is generally only slightly higher than the recommended operating range of the inverter. Since the solid-state inverter is not built to have surge capability, any power demand in excess of the rated maximum could destroy the inverter in a small fraction of a second. Conventional protection circuits are known in the art to protect against such a result when an overload condition occurs, but such circuits do not enable the inverter to supply significantly higher output power during such an overload condition.
The solid-state inverter's lack of an ability to handle power surge demands results in a severe mismatch between the rating of the solid-state inverter and the load. For example, a one horsepower induction motor can require 4,000 watts of power when power is first applied to the motor, but only 1,000 watts of power after the motor has been running for a short time. To avoid destruction, a conventional solid-state inverter would need to have a rated load capacity of 4,000 watts to power such a motor rather than the 1,000 watts of power needed by the motor after it has started running. Significant economies are possible if a lower rated inverter could be used in a given application, in terms of inverter component size, etc.
Prior art attempts have been made to build inverters having greater overload or surge capacity. Inverters built of thyristors, for example, have been attempted, but these circuits have not significantly enhanced the surge capability of such inverters. See, e.g., U.S. Pat. No. 4,225,912 to Messer.